For example, if you ask “does the Colossus of Rhodes exist?” the correct answer should be “it did, but it doesn’t now. So, in that sense: no, the past and future don’t exist (by definition). So if things in the past and future exist in the exact same way that things in the present exist, then doesn’t that mean that they’re fixed? Even if the future does exist, it doesn’t mean that events are set in stone in any useful or important way. That wave function is set in stone by an extant future, but that doesn’t tell you exactly what will happen.